Volcanoes

Volcanoes are one of Earth’s most terrifying—and exhilarating—spectacles. These vents in the ground issue lava, ash, and noxious gases at temperatures as high as 2,100°F (1,150°C) and are the closest we’ve ever gotten to the churning engine that keeps our planet running. What causes them to erupt?

Magma, or molten rock, collects in chambers beneath Earth’s surface. As magma accumulates, pressure in the magma chamber increases. If the pressure gets high enough, overlying rocks break, allowing the magma to escape as lava.

Watch this video about how scientists research volcanic activity at Mt. Etna, in Italy.

Krakatoa, Indonesia

August 26, 1883

When this volcanic island exploded and collapsed into the sea, the world felt the effects for years.

What happened?

Ash and steam had been erupting from Krakatoa since May, when a series of cataclysmic explosions began on August 26.

The final climactic blow the following day blew the island to bits. The blast was heard in Africa, almost 3,000 miles (5,000 kilometers) away.

When the tower of ash from the explosion collapsed, it sent a 130-foot (40-meter) tsunami surging onto neighboring coastlines. More than 36,000 people lost their lives in Java, Sumatra, and surrounding islands.

Krakatoa and other large, explosive volcanoes can temporarily alter global climate by injecting sulfurous gases into the high atmosphere. There, the gases react with water vapor to produce droplets of sulfuric acid. The droplets shade, and thus cool, the planet before settling out within two or three years.

Mount Vesuvius, Italy

August 24, AD 79

This painting of a scene from Pompeii is by Karl Pavlovich Bryullov (1799-1852).

Mt. Pelée, Martinique

May 8, 1902

St. Pierre, a city on the island of Martinique that was known as the Paris of the Caribbean, was completely destroyed by the nearby volcano, Mt. Pelée.

What happened?

A dense, swirling cloud of hot gases, volcanic ash, and rocks—known as nuée ardente—formed when the eruption column of the volcano collapsed downward. Moving at tremendous speed, it incinerated everything in its path.

The May 8 flow raced straight for the center of St.-Pierre at about 300 miles (500 kilometers) an hour.

About 30,000 people died in St. Pierre within the space of two minutes. (Only two survived.)

News of the disaster horrified the world. Geologists, anxious to understand the science behind the tragedy, were drawn to Martinique.

Dents in the sign (indicated by arrows) were likely made by small pebbles within the high-speed, intensely hot pyroclastic flow.

Many explosive volcanoes are located far from major population centers, but worldwide, several hundred million people make their homes near volcanoes that have previously erupted—and could do so again. Fortunately, volcanoes generally show signs of activity weeks to days before erupting, and continuous monitoring of active volcanoes has substantially reduced risks. Fewer than 1,600 people have died in volcanic eruptions in the last 25 years.

Yet mega-eruptions, which can spew thick layers of ash over thousands of square miles, may occur only once in several tens of thousands of years. Because they are so infrequent, the risks associated with such events are poorly understood.